1. Field of the Invention
The present invention generally relates to cooling an electronic system such as a computer system. More particularly, the present invention relates to a technique for cooling a computer while reducing fan noise. Still more particularly, the invention relates to controlling the operation of a chassis fan based on the operation of another fan (e.g., a CPU fan).
2. Background Information
Computer systems include numerous electrical components that draw electrical current to perform their intended functions. A computer's microprocessor or central processing unit (“CPU”) requires electrical current to perform many functions such as controlling the overall operations of the computer system and performing various numerical calculations. Any electrical device through which electrical current flows produces heat. The amount of heat any one device generates generally is a function of the amount of current flowing through the device.
Typically, each manufacturer designs its products to operate correctly within a predetermined temperature range. If the temperature exceeds the predetermined range (i.e., the device becomes too hot or too cold), the device may not function correctly thereby potentially degrading the overall performance of the computer system. Thus, it is desirable for a computer system generally, and its components specifically, to operate within a thermally benign environment.
A conventional computer system includes one or more fans to blow air over the electronics to keep the electronics cool. A fan's ability to cool a computer is a function of the volume and speed of the airflow it is able to create. Faster fans generally provide greater cooling ability than slower fans. Fans, however, also make noise and the noise level is a function of the speed of the fan; faster fans are noisier than slower fans.
In some computers having a single fan, the fan is turned on when the computer is turned on and operated at a predetermined speed until the computer is turned off. Although very simple and satisfactory for cooling purposes, this approach results in the fan continuously making a great deal of noise, which is generally annoying to the operator. In other computers, to reduce fan noise, upon boot up, the fan initially is activated at a relatively slow speed and spun faster only if it is determined (using, for example, a thermal sensor) that the computer has become hotter. Once an increase in temperature is detected, the fan speed is increased. This approach recognizes that a computer that has just booted up and does not yet generate much heat does not need its fan to spin at a high speed. Such a computer may include one or more temperature sensors that are monitored to determine the thermal state of the computer. If it is determined that the computer is becoming too hot, the fan's speed can be increased. Although increasing fan speed disadvantageously increases the sound level of the computer, generally it is a necessary act to keep the computer from overheating.
Early microprocessors dissipated a couple of watts of energy. Today's microprocessors dissipate tens of watts (e.g., 60 watts) and thus become considerably hotter than early microprocessors. In fact, today's microprocessors typically require a dedicated fan whose primary purpose is to cool the microprocessor (also called a “CPU”). Such computers typically also have an additional chassis fan whose purpose is to help cool other portions of the computer such as most of the interior volume of the chassis.
Some techniques for fan speed control involve the computer's basic input/output system (“BIOS”) code. The BIOS generally is low level code which provides control over various low level functions in the computer. One such function is fan speed. Thus, BIOS code can be executed to change the speed of one or more of the computer's fans. Executing BIOS code in this manner, however, generally requires the operating system to be temporarily interrupted while the BIOS code runs. Interrupting the operating system detrimentally impacts overall system performance and thus is undesirable.
With two fans in a computer system, the noise level from the computer may become annoyingly loud. As a result, there is a need to provide a cooling system for a computer that minimizes the sound level of the fans as much as possible. These two design goals are generally at odds with each other—a fan capable of keeping a computer sufficiently cool generates a great deal of noise and a quiet fan may be unable to sufficiently cool the computer, all else being equal. Accordingly, there is a need to provide a cooling system for a computer that effectively trades off system cooling and fan noise. At the same time, it is desirable to reduce the cost and complexity of a computer system. Thus, the provision of additional benefits and features preferably should not require additional circuitry. Moreover, it is desirable to provide adequate cooling, while minimizing sound level and system complexity and also maintaining the computer at as high a performance level as possible.